Om det kommer åska i kväll, stannar vi hemma i stället för att gå till stranden.

Om means if and introduces a condition:

• Om det kommer åska i kväll = If there is thunder tonight / If a thunderstorm comes tonight

So the first part sets up the condition, and the second part tells you what happens if that condition is true.

Here det is a dummy subject, similar to English it in weather expressions:

• Det regnar = It is raining
• Det snöar = It is snowing

In Swedish, weather and other impersonal expressions often need det even though it does not refer to a specific thing.

So det kommer åska is a natural weather-style expression.

Literally, kommer means comes, but in this sentence it works more like:

• there will be
• a thunderstorm is coming
• it’s going to thunder

Swedish often uses kommer in expressions about approaching weather:

• Det kommer regn = Rain is coming
• Det kommer åska = Thunder/a thunderstorm is coming

So it is not just a literal movement verb here; it helps describe incoming weather.

Here åska is understood as a noun, meaning thunder or a thunderstorm.

So:

• det kommer åska = there will be thunder / a thunderstorm is coming

Swedish also has the verb att åska = to thunder, but that is a different structure:

• Det åskar = It’s thundering

Both are possible in Swedish, but this sentence uses the noun-based weather expression.

This is because Swedish follows the V2 rule in main clauses: the finite verb usually comes in the second position.

The sentence begins with a subordinate clause:

• Om det kommer åska i kväll

After that, the main clause starts, and Swedish puts the verb first in that clause:

• stannar vi hemma

Compare:

• Vi stannar hemma. = normal word order
• Om det kommer åska i kväll, stannar vi hemma. = inversion after the opening clause

This is very common in Swedish.

Swedish distinguishes between:

• hem = homeward / to home (movement)
• hemma = at home (location)

Here the meaning is staying in one place, so Swedish uses hemma:

• Vi stannar hemma = We stay home / We stay at home

If you were talking about going home, you would use hem:

• Vi går hem = We go home

I stället för att means instead of when followed by a verb.

Structure:

• i stället för att + infinitive

So:

• i stället för att gå = instead of going

In this sentence:

• stannar vi hemma i stället för att gå till stranden
• we stay home instead of going to the beach

A useful comparison:

• i stället för kaffe = instead of coffee
• i stället för att dricka kaffe = instead of drinking coffee

So att appears when a verb follows.

Because gå is an infinitive verb, and after i stället för Swedish normally uses:

• att + infinitive

So:

• i stället för att gå = instead of going

This is a fixed and very common pattern.

Yes, both i kväll and ikväll are used.

• i kväll = slightly more clearly separated into in/this evening
• ikväll = very common as a single-word adverb

Both are accepted in modern Swedish, and the difference is usually small. In many contexts, they are interchangeable.

So this sentence could also be written:

• Om det kommer åska ikväll, stannar vi hemma i stället för att gå till stranden.

Stranden is the definite form of strand:

• en strand = a beach
• stranden = the beach

So:

• gå till stranden = go to the beach

Swedish often uses the definite form where English also uses the.

Yes. Swedish has several natural ways to express this idea, for example:

• Om det kommer åska i kväll = if thunder is coming tonight
• Om det ska åska i kväll = if it is going to thunder tonight
• Om det åskar i kväll = if it thunders / if it’s thundering tonight

They are all possible, though the nuance changes a little:

• kommer åska focuses on approaching weather
• ska åska sounds a bit like a forecast
• åskar describes the thunder itself happening

The original sentence is perfectly natural.